Growth of crystal silicon ribbon, also known as silicon dendritic web, is currently established through use of a crucible/susceptor system, the crucible containing molten silicon from which the silicon ribbon is pulled. The present crucibles used in dendritic silicon web growth are much longer than they are wide and, in order to minimize convective flow, are shallow. They are made from thin-walled [0.080 in. (0.2 cm.)] semiconductor grade quartz. The crucibles typically are contained in a cavity in a molybdenum susceptor. Because control of the growth process is based upon subtle temperature changes, the quartz thickness of the crucible is held to a minimum to miximize the rate of heat transfer from the susceptor to the melt. This minimum thickness, however, presents the disadvantage of crucible wall collapse under operating conditions, impeding the heat transfer sought to be maximized.
Continuous growth of single crystal silicon ribbon requires that the silicon melt be replenished with silicon as the crystal is being pulled. Under ideal conditions, the replenishment is added at the hottest location of the melt; and the crystal is grown from the coolest location, which coincides with geometric center of the melt. Any region of the melt that is cooler than the location of the growing crystal may nucleate the freezing of the silicon and interfere with or terminate crystal growth. For this reason, it is important that heat transfer be uniform throughout the melt.
Post-run observations of cold charges show the inward collapse of crucible side walls, which interferes with uniform heat transfer to the melt. It was previously believed that crucible side wall collapse occurred during furnace cool down. However, an experiment was conducted with special slotted lids that showed that the crucible walls pulled inward during the melting of the silicon (at 1420.degree. C., above the softening point of quartz). This is believed to be the combined result of high molten silicon surface tension forces acting on the softened quartz side walls and, once equilibrium has been disturbed, gravity.
The depth of the silicon melt, the height, thickness, and length of the unsupported crucible wall, and the forces of gravity and the surface tension of molten silicon acting on the wall at a temperature above the softening point of quartz determine the magnitude and direction of wall movement. The analysis of the bending of the crucible wall during web growth is in fact a complicated problem involving distributed forces, plastic flow, and creep.
FIG. 1 shows a sketch of a "collapsed" crucible as it would be presumed to appear at operating temperatures. The inward movement of the crucible side wall 10 would be greatest at the mid-point 10a of the longest unsupported span. Typically, this is in the middle of the long dimension of the crucible directly across from the growing web, and most likely, in an undercooled region of the melt.
When the side walls of the crucible are pulled inward and away from the susceptor heat source, not only does the transfer of heat to the upper regions of the melt become impeded, but the temperature at the inner face of the melt with the crucible side wall lowers and becomes a potential nucleation point for the formation of silicon "ice". A number of runs having poor crystal growth due to frequent "ice" formation appeared to also exhibit severe crucible collapse (post-run observation). The degree of crucible wall collapse is quite variable and influenced by the position of the crucible in the susceptor, the amount of silicon in the crucible, and the location of the crucible/susceptor interfaces relative to the location of the induction coil. Variable wall collapse will not give reproducible thermal conditions for crystal growth. It would, therefore, constitute an improvement over current crystal silicon ribbon growth technology to develop a crucible/susceptor system that would not exhibit severe crucible collapse.